High performance field effect transistors comprising carbon nanotubes fabricated using solution based processing

ABSTRACT

The present invention is directed toward field effect transistors (FETs) and thin film transistors (TFTs) comprising carbon nanotubes (CNTs) and to methods of making such devices using solution-based processing techniques, wherein the CNTs within such devices have been fractionated so as to be concentrated in semiconducting CNTs. Additionally, the relatively low-temperature solution-based processing achievable with the methods of the present invention permit the use of plastics in the fabricated devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to commonly assigned co-pending U.S.patent application Ser. No. 10/925,312, filed Aug. 24, 2004,incorporated by reference herein.

TECHNICAL FIELD

The present invention relates generally to methods for preparing fieldeffect transistors (FETs), and more specifically to FETs comprisingcarbon nanotubes and to methods for preparing such FETs.

BACKGROUND INFORMATION

Organic-based electronic devices, in which the active semiconductingmaterial is an organic compound (e.g., pentacene) or conjugated polymer(e.g., poly(3-alkylthiophene)), are attractive for their relativelyinexpensive, low-temperature processability. However, such organic-baseddevices generally have limited application due to the low field effectmobilities (μ) realized in such devices and their poor reliability(e.g., reproducibility, variable threshold voltage, air stability, andprocessability). Different organic materials have been screened intraditional organic thin film transistors (TFTs), yet the mobilities insuch materials are generally not much greater than about 1 cm²/Vsec (C.Dimitrakopoulos et al., “Organic Thin Film Transistors for Large AreaElectronics,” Adv. Mater., 2002, 14(2), 99-117). While organic TFTs withfield effect mobilities of 15 cm²/Vsec have been reported with pentacenedevices on treated silicon via evaporation, such results have not beenreproducible (T. Kelly et al., ACS ProSpective Meeting, Thin-FilmElectronics: Materials, Devices, and Applications, Jan. 25-28, 2004,Miami, Fla.). Single crystals of organic materials have been shown tohave mobilities as high as 10-15 cm²/Vsec, but they are difficult toproduce and still do not approach the mobility of 100 cm²/Vsecanticipated to be required for high-performance devices and circuits (V.Sundar et al., “Elastomeric Transistor Stamps: Reversible Probing ofCharge Transport in Organic Crystals,” Science, 2004, 303, 1644-1647).Thus, high-performance field effect transistors (FETs) on plastic cannotbe achieved with traditional organic materials, or, for that matter,with silicon, since in the latter case processing methods are limited toamorphous silicon.

It would be highly desirable to be able to manufacture high mobilityTFTs with low-temperature, solution-based processing techniques thatwould allow low cost, high-performance devices for large areaelectronics. Indeed, such a processing method has been a long-soughtafter goal as higher mobilities would allow faster switching in high-enddisplays and permit logic applications (C. Dimitrakopoulos et al.,“Organic Thin Film Transistors for Large Area Electronics,” Adv. Mater.,2002, 14(2), 99-117). Hence, the problem to be overcome is three-fold:limited processing capability, poor performance, and poor reliability oftraditional organic TFTs.

There has been considerable effort to overcome the above-describedproblems by replacing the semiconducting material in such TFTs with acarbon nanotube (CNT) network (Baughman et al., Carbon Nanotubes—theRoute Toward Applications,” Science, 2002, 297, 787-792; Dai, “CarbonNanotubes: Synthesis, Integration, and Properties,” Acc. Chem. Res.,2002, 35, 1035-1044). While CNT transistors based on the use of a singleCNT per channel are not currently commercializable due to expensive,unreliable, and uncontrollable processes, CNT transistors based on CNTnetworks have been prepared by growing CNT networks on siliconsubstrates at high temperatures, temperatures that are not compatiblewith plastic substrates. Transport properties of such single-wall carbonnanotube (SWNT) network transistors have been reported as havingmobilities of 10 cm²/Vsec and I_(on)/I_(off) of 10⁵. At higher SWNTdensities, mobilities of 100 cm²/Vsec are obtained, but with a high offcurrent (I_(off)) (Snow et al., “Random Networks of Carbon Nanotubes asan Electronic Material,” Appl. Phys. Lett., 2003, 82(13), 2145-2147).Furthermore, such in situ growth of CNTs directly on substrates providesan uncontrollable mixture of semiconducting and metallic tubes such thaton/off ratios of such devices are poor (Xiao et al., “High-MobilityThin-Film Transistors Based on Aligned Carbon Nanotubes,” Appl. Phys.Lett., 2003, 83, 150-152). Such in situ growth processes also tend to below yield processes.

Martel et al. (Appl. Phys. Lett., 1998, 73, 2447) prepared single-tubedevices by dispersing a dilute suspension of CNTs onto a substrate andthen patterned electrodes on the surface comprising the CNTs. However,this process is not practical, as described therein, and the metallicCNTs still present can lead to short-circuited devices.

The fabrication of TFTs based on single-wall carbon nanotube (SWNT)networks has been accomplished on silicon substrates from which theywere then transferred to plastic substrates (Bradley et al., “FlexibleNanotube Electronics,” Nano Lett., 2003, 3(10), 1353-1355). Such atransfer process is not practical or cost-effective, however, andgrowing tubes (individual or network) on plastic is not possible due tothe high temperatures typically required.

DNA-streptavidin complexes have been used to assemble templated CNT FETsusing single, isolated semiconducting CNTs (Keren et al., “DNA-TemplatedCarbon Nanotube Field-Effect Transistor,” Science, 2003, 302,1380-1382). However, as already mentioned, such single CNT devices arenot practical and such methods still require isolation of semiconductingCNTs.

Efforts to overcome processing limitations of in situ CNT growth for FETdevices have led some to fabricate silicon-based nanowires on plasticsubstrates (McAlpine et al., “Nanoimprint Lithography for Hybrid PlasticElectronics,” Nano Lett., 2003, 3(4), 443-445; Duan et al.,“High-Performance Thin-Film Transistors Using Semiconductor Nanowiresand Nanoribbons,” Nature, 2003, 425, 274-278; McAlpine et al.,“High-Performance Nanowire Electronics and Photonics on Glass andPlastic Substrates,” Nano Lett., 2003, 3(11), 1531-1535). Such nanowiresare limited, as they are typically produced in very low yield andreadily oxidize in air. Furthermore, inorganic nanowires suffer fromtrapped states on the nanowire surface and difficulties in doping.

Several recent publications have described processes to separatesemiconducting SWNTs from metallic SWNTs (D. Chattopadhyay et al., “ARoute for Bulk Separation of Semiconducting from Metallic Single-WallCarbon Nanotubes,” J. Am. Chem. Soc., 2003, 125, 3370; M. Zheng et al.,“Structure-Based Carbon Nanotube Sorting by Sequence-Dependent DNAAssembly,” Science, 2003, 302, 1545-1548; Weisman, “Four Degrees ofSeparation,” Nat. Mater., 2003, 2, 569-570), yet no one has used theseprocesses to fabricate TFT based on solution-cast SWNT networks that areenriched with semiconducting SWNTs.

Selective chemistry to render the metallic SWNTs non-conducting has beendeveloped. Such chemistry selectively reacts metallic SWNTs in thepresence of semiconducting SWNTs. Such chemistry disrupts theconjugation of the metallic SWNTs and effectively destroys theirmetallic character (M. Strano et al., Science, 2003, 301, 1519).Recently, this approach has been used to fabricate FETs comprising CNTsgrown in situ on a device platform (L. An et al., “A Simple ChemicalRoute to Selectively Eliminate Metallic Carbon Nanotubes in NanotubeNetwork Devices,” J. Am. Chem. Soc., 2004, 126(34), 10520-10521), butsuch processing still requires high temperatures to generate the CNTsand leaves chemically-destroyed metallic CNTs in the device and thischemistry is acknowledged by the authors to not be completely selective.

In light of the above, a method to inexpensively manufacture FETs, andTFTs in particular, at low temperatures using both solution-basedprocessing and CNTs to provide high field effect mobilities would behighly desirable, as it would permit such devices to be fabricated withplastic substrates. Such resulting low-cost devices would allow theirincorporation into articles of manufacture heretofore economicallyunrealizable.

BRIEF DESCRIPTION OF THE INVENTION

Some embodiments of the present invention are directed toward fieldeffect transistors (FETs) comprising carbon nanotubes (CNTs) and tomethods of making such devices using solution based processingtechniques, wherein the CNTs within such devices have been fractionatedso as to be concentrated in semiconducting CNTs. Additionally, therelatively low temperature solution-based processing methods permit theuse of plastics in the fabricated devices.

Generally, such FET devices comprise: 1) a substrate; 2) a gateelectrode; 3) a dielectric layer in contact with the gate electrode; 4)a semiconducting active material in contact with the dielectric layer,wherein the semiconducting active material comprises carbon nanotubes,the carbon nanotubes having been non-destructively enriched insemiconducting carbon nanotubes; and 5) source and drain electrodes incontact with the semiconducting active material.

Generally methods of making such FET devices comprise the steps of: 1)dispersing carbon nanotubes in a solvent comprising a fractionatingagent to form a dispersion; 2) centrifuging the dispersion to effect,with the aid of the fractionating agent, a fractionation of carbonnanotubes by electronic type into sediment and supernatant formed by thecentrifuging, such that the supernatant becomes enriched insemiconducting carbon nanotubes; and 3) transferring the carbonnanotubes from the supernatant to a substrate to serve as activesemiconducting material in a field effect transistor.

Many embodiments of the present invention offer advantages over theprior art including low temperature solution-based processing, and theuse of carbon nanotubes to afford enhanced performance relative totraditional organic-based FETs. Low temperature solution-basedprocessing methods permit the use of a broad range of materials in theprocessing of FET devices in accordance with some embodiments of thepresent invention. Furthermore, use of CNTs overcomes many of theperformance and operating limits of organic-based FETs, whilemaintaining solution based processability. Finally, the fractionationtechniques described herein can enable a concentrating of semiconductingCNTs within a given CNT population capable of affording superiorproperties when incorporated into a FET device.

While much of the discussion herein is directed at single-wall carbonnanotubes (SWNTs), it will be understood by those of skill in the artthat the scope of such exemplary embodiments can be extended to includemulti-wall carbon nanotubes (MWNTs) and particularly double-wall carbonnanotubes.

In accordance with some embodiments of the present invention,applications for the devices include, but are not limited to, liquidcrystal displays (LCDs), organic light emitting diode (OLED) displays,radiofrequency identification (RFID), sensors, and X-ray detectors.While some applications involve a mere replacement of existing FETdevices, new applications for such FET devices will present themselvesas a result of their high performance and processing flexibility.

The foregoing has outlined rather broadly the features of the presentinvention in order that the detailed description of the invention thatfollows may be better understood. Additional features and advantages ofthe invention will be described hereinafter which form the subject ofthe claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a TFT device in accordance with embodiments of thepresent invention;

FIG. 2 illustrates another TFT device in accordance with embodiments ofthe present invention;

FIG. 3 is a flow diagram illustrating embodiments by which FET devicesof the present invention can be fabricated;

FIG. 4 is a flow diagram illustrating the steps involved infractionating CNTs to provide CNT populations enriched in semiconductingCNTs;

FIG. 5 is a 514 nm Raman spectra of SWNTs solvent-cast onto quartz fromCHCl₃;

FIG. 6 shows a radial breathing mode (RBM) region 633 nm Raman spectraof SWNTs solvent-cast onto quartz from CHCl₃;

FIG. 7 shows a tangential band region 633 nm Raman spectra of SWNTssolvent-cast onto quartz from CHCl₃;

FIG. 8 is an UV/vis/NIR absorption spectrum showing the attachment ofthe polymer P3HT with SWNTs;

FIG. 9 is an UV/vis/NIR absorption spectrum of pristine or untreatedSWNTs in CHCl₃;

FIG. 10 illustrates yet another TFT device in accordance withembodiments of the present invention;

FIG. 11 illustrates a TFT testing device in accordance with Examples5-7;

FIG. 12 is a cross-sectional side view of region 1200, identified bydashed borders in FIG. 11;

FIG. 13 is an AFM image depicting SWNT density on a substrate, accordingto an embodiment of the present invention;

FIG. 14 is a transfer curve for the device in Example 6;

FIG. 15 is an SEM image of SWNTs dispersed onto the testing device ofExample 5;

FIG. 16 is an SEM image depicting a higher magnification of one portionof the area shown in FIG. 15; and

FIG. 17 depicts a transfer curve for the device in Example 7.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, specific details are set forth such asspecific quantities, sizes, etc. so as to provide a thoroughunderstanding of embodiments of the present invention. However, it willbe obvious to those skilled in the art that the present invention may bepracticed without such specific details. In many cases, detailsconcerning such considerations and the like have been omitted inasmuchas such details are not necessary to obtain a complete understanding ofthe present invention and are within the skills of persons of ordinaryskill in the relevant art.

Referring to the drawings in general, it will be understood that theillustrations are for the purpose of describing a particular embodimentof the invention and are not intended to limit the invention thereto.

In some embodiments, the present invention is directed toward fieldeffect transistors (FETs) comprising carbon nanotubes (CNTs) and tomethods of making such devices using solution based processingtechniques, wherein the CNTs within such devices have been fractionatedso as to be concentrated in semiconducting CNTs. Additionally, therelatively low temperature solution-based processing achievable with themethods of the present invention permits the use of plastics in thefabricated devices, particularly wherein such devices are thin filmtransistors (TFTs).

A “field effect transistor,” as used herein and abbreviated “FET” is athree-terminal semiconductor device in which the current flow throughone pair of terminals, the source and the drain, is controlled ormodulated by an electric field that penetrates the semiconductor; thisfield is introduced by the voltage applied at the third terminal, thegate.

A “thin film transistor,” as used herein and abbreviated “TFT” is an FETthat is fabricated using thin-film techniques generally on an insulatingsubstrate rather than on a semiconductor chip. In some embodiments, theinsulating substrate is silicon with an oxide surface layer. Theinsulating substrate reduces the bulk capacitance of the device andhence the operating speed can be increased.

“Carbon nanotubes,” as used herein and abbreviated “CNTs,” refer toall-carbon tubes or cylinders having diameters on the order of about 0.5nm to about 100 nm. “Single-wall carbon nanotubes,” abbreviated “SWNTs”are but a single graphene sheet rolled upon itself to form such acylinder (Iijima et al., “Single-Shell Carbon Nanotubes of 1-nmDiameter,” Nature, 1993, 363, 603-605), whereas “multi-wall carbonnanotubes,” abbreviated “MWNTs” are comprised of a plurality of suchrolled sheets concentrically arranged like a Russian nesting doll(Iijima, “Helical Microtubules of Graphitic Carbon,” Nature, 1991, 354,56-58), the simplest being a double-wall carbon nanotube. The diameterand helicity of such nanotubes can be described by a roll-up vector(n,m) (Dai, “Carbon Nanotubes: Synthesis, Integration, and Properties,”Acc. Chem. Res., 2002, 35, 1035-1044). When ln-ml=3 k, where k is aninteger or zero, the nanotube is metallic. All other combinations of nand m result in semiconducting nanotubes.

“Plastics,” as used herein, generally refers to all processable forms ofpolymeric material, such polymeric material including, but not limitedto, thermoplastics, thermosets, elastomers, and combinations thereof.“Polymeric material,” is further defined to encompass polymer precursormaterials (e.g., monomers), oligomers, epoxy resins, cyanate resins, anddendritic materials.

“Solvent-casting,” or “solution-casting,” as used herein, generallyrefers to the deposition of a carbon nanotube-containing solution orsuspension onto a substrate. “Spin-casting,” as used herein, generallyrefers to the deposition of such solutions/suspensions onto a spinning(i.e., rotating) substrate.

Generally, field effect transistors of the present inventioncomprise: 1) input and output electrodes; and 2) a semiconducting activematerial coupled to the input and output electrodes, wherein thesemiconducting active material comprises carbon nanotubes, the carbonnanotubes having been non-destructively enriched in semiconductingcarbon nanotubes.

Depending on the embodiment, the dimensions of such FET devices of thepresent invention can be varied considerably. Typically, the size ofsuch devices can vary such their channel widths are between about 50 nmand about 1 mm, and their channel lengths are between about 50 nm andabout 100 μm.

As a purely semiconducting population of CNTs cannot be synthesizeddirectly (i.e., such synthesis procedures always yield mixtures ofmetallic and semiconducting CNTs), enrichment of a CNT mixture in eithermetallic or semiconducting concentration must be carried outpost-synthesis. While some such methods of enrichment rely on selectivechemical reactivity of metallic CNTs, these techniques are “destructive”in that they chemically alter the metallic nanotubes. The presentinvention relies primarily on non-destructive enrichment techniques tofractionate populations of CNTs. Such non-destructiveenrichment/separation is described in commonly assigned co-pending U.S.patent application Ser. No. 10/925,312, filed Aug. 24, 2004.

More specifically, the FET devices of the present invention cancomprise 1) a substrate; 2) a gate electrode; 3) a dielectric layer incontact with the gate electrode; 4) a semiconducting active material incontact with the dielectric layer, wherein the semiconducting activematerial comprises carbon nanotubes, the carbon nanotubes having beennon-destructively enriched in semiconducting carbon nanotubes; and 5)source and drain electrodes in contact with the semiconducting activematerial. In some embodiments of the present invention, the FET devicesof the present invention are TFTs.

Examples of TFT devices, in accordance with some embodiments of thepresent invention, are shown in FIGS. 1 and 2. Referring to FIG. 1, TFTdevice 100 comprises a substrate 101 in contact with gate electrode 102and dielectric layer 103. Still referring to FIG. 1, a semiconductinglayer 104 resides on dielectric layer 103, and source and drainelectrodes 105 and 106, respectively, reside on semiconducting layer104. Referring to FIG. 2, TFT device 200 comprises a substrate 201 incontact with a gate electrode 202 and a dielectric layer 203. Residingon dielectric layer 203 are source and drain electrodes 205 and 206,respectively, as well as semiconducting layer 204. Other suitableconfigurations exist, as will be appreciated by those of skill in theart.

Substrates for devices of the present invention may be plastic. Suchsubstrates, are not, however, limited to plastic. Substrates can be ofany material or combinations of material that suitably provides for asubstrate in accordance with the present invention. Other such substratematerials include, but are not limited to, silicon, glass, plastic,metal foil, ceramic, and combinations thereof.

Examples of gate electrode materials include, but are not limited to,gold, platinum, molybdenum, carbon nanotubes, conducting polymers (withand without carbon nanotubes), and combinations thereof.

Exemplary dielectric materials include, but are not limited to, SiO₂,nitrides, spin-on-glass, polyimides, polyvinylphenol, parylene, andcombinations thereof.

As noted above, the semiconducting active material comprises CNTs. SuchCNTs are generally mixtures of metallic and semiconducting CNTs thathave been enriched in semiconducting nanotubes. Such CNTs can be SWNTs,MWNTs, and combinations thereof. In some embodiments, such CNTpopulations may comprise chemically-functionalized CNTs.

The lengths of the CNTs within such a semiconductive active layer mayvary from about 20 nm to about 100 μm, but other lengths are possibleoutside this range. The choice of particular lengths or ranges oflengths is, at least in part, dependent upon the dimensions of thedevice(s) they are being incorporated into, particularly the distancebetween the source and drain electrodes.

In addition to CNTs, the semiconductor active region may furthercomprise one or more different polymeric materials, wherein thepolymeric materials can be conductive polymers, non-conductive polymers,or combinations thereof. Such polymeric materials can be conjugated,non-conjugated, or combinations thereof. In some embodiments, the CNTsare dispersed in a polymeric matrix comprising such above-describedpolymeric material. Such polymeric materials include, but are notlimited to, polythiophenes, polythiophene derivatives,poly(bithiophene-fluorenes), and combinations thereof. In someembodiments, dendritic materials are attached to such polymericmaterials, i.e., dendrimers attached to a polymer backbone at one ormore locations, typically one or both ends. Suitable dendritic materialincludes, but is not limited to, aliphatic ethers, aliphatic esters,aliphatic amines, aliphatic amides, aromatic ethers, aromatic esters,aromatic amines, aromatic amides, aromatic alkynes, phenylenes, aromaticalkenes, polyether carbamates, and combinations thereof.

In some embodiments, additional materials are added to the CNTs and/orpolymeric material in the semiconducting active layer. Such additionalmaterials include, but are limited to, polymeric material, oligomers,single-stranded DNA, polyethylenimine, C₆₀, aromatic organic molecules,and combinations thereof.

In some embodiments, the additional materials and/or polymeric materialserves to dope or further dope the CNTs within the semiconducting activeregion. Such doping can be of the n-type or p-type.

Generally, the semiconducting active region comprises a CNT density thatis at or above a percolation threshold, wherein a percolation thresholdis defined as the minimum density of nanotubes required to obtain aconductive path between the source and drain electrodes.

The FET devices of the present invention comprising CNTs in thesemiconducting active layer or region generally have field effectmobilities of about 10⁻³ cm²/Vsec to about 10⁵ cm²/Vsec, and typicallyhave field effect mobilities of about 0.1 cm²/Vsec to about 30 cm²/Vsec.Such devices further typically have current modulation (i.e.,I_(on)/I_(off) ratios) of about 10 to about 10⁵.

It is important to note that in some embodiments, the FET devices of thepresent invention have been fabricated with materials generally notfound in such devices (e.g., plastics), and that such devices made withsuch materials exceed the performance characteristics generally found inorganic TFT devices. Such devices are made possible by thesolution-based processing described below.

Applications for the above-described devices include, but are notlimited to, liquid crystal displays (LCDs), organic light emitting diode(OLED) displays, radiofrequency identification (RFID), sensors, andX-ray detectors.

Generally, methods of the present invention include suspending CNTs in asolvent to form a suspension, and solvent-casting the suspension onto asubstrate, wherein the CNTs serve as a bridging network between sourceand drain electrodes of a FET device.

In some embodiments, methods of making such FET devices comprise anumber of steps. Referring to FIG. 3, the steps are: (Step 3001)dispersing carbon nanotubes (i.e., a mixture of semiconducting andmetallic CNTs) in a solvent comprising a fractionating agent to form adispersion; (Step 3002) centrifuging the dispersion to effect, with theaid of the fractionating agent, a fractionation of carbon nanotubes byelectronic type into sediment and supernatant formed by thecentrifuging, such that the supernatant becomes enriched insemiconducting carbon nanotubes; and (Step 3003) transferring the carbonnanotubes from the supernatant to a substrate to serves as activesemiconducting material in a field effect transistor. It is an advantageof the present invention that such methods can be used in thefabrication of devices on plastic substrates and/or with plasticcomponents.

Solvents suitable for dispersal of CNTs include, but are not limited to,N,N-dimethylformamide (DMF), chloroform (CHCl₃), o-dichlorobenzene(ODCB), dichloromethane (CH₂Cl₂), benzene, toluene, xylenes, mesitylene,dimethylsulfoxide (DMSO), water, and combinations thereof. In someembodiments, a surfactant is used to facilitate the dispersion. In someembodiments, ultrasonication and/or some form of mechanical agitation isutilized to facilitate such dispersal. In some embodiments, heat isapplied to facilitate dispersal.

Fractionating agents, according to the present invention, are speciesthat complex or associate (in a non-covalent, non-destructive way) withthe carbon nanotubes in the dispersion so as to effect a fractionationof the CNTs by type (i.e., metallic and semiconducting) whencentrifuged, and provide for a supernatant that is enriched insemiconducting carbon nanotubes. In some embodiments, the fractionatingagents are polymeric. In some embodiments these polymeric fractionatingagents are amphiphilic. In some embodiments, the polymeric fractionatingagents are conductive polymers. Suitable polymers include, but are notlimited to, polythiophenes, polythiophene derivatives,poly(bithiophene-fluorenes), single-stranded DNA, and combinationsthereof. Suitable polythiophenes include, but are not limited to,poly-3-hexyl-thiophene (P3HT), the structure of which is shown below.

In some embodiments, the fractionating agent assists in the de-bundlingand dispersion of CNTs. CNTs, especially SWNTs, tend to agglomerate intobundles or ropes held together by van der Waals forces. In suchembodiments, there is generally an energetic preference for CNT contactto the fractionating agent over CNT contact to itself. Such dispersionand fractionalization are described in commonly assigned co-pending U.S.patent application Ser. No. 10/925,312, filed Aug. 24, 2004.

In some embodiments, the fractionating agents comprise dendriticmaterial. Such dendritic material may be attached to other polymericmaterial. Suitable dendritic material includes, but is not limited to,aliphatic ethers, aliphatic esters, aliphatic amines, aliphatic amides,aromatic ethers, aromatic esters, aromatic amines, aromatic amides,aromatic alkynes, phenylenes, aromatic alkenes, polyether carbamates,and combinations thereof.

Centrifugation typically is carried out in a centrifuge tube at a speedbetween about 1000 rpm and about 5000 rpm for a duration between about 1minute and about 20 minutes. Such centrifugation typically yieldssediment and supernatant, the sediment having gravitated to the bottomof the centrifuge tube and the supernatant being the liquid on top. Itis in these two phases that a fractionation of the CNT mixture isrealized: the sediment being enriched in metallic CNTs and thesupernatant being enriched in semiconducting CNTs-relative to thestarting material.

In some embodiments, transfer of the supernatant to a substrate firstinvolves a separation of the CNTs within the supernatant from the CNTsin the sediment. Typically this is done with simple decanting, or byoptional filtration.

In embodiments wherein the fractionating agent is a polymer, theoptional filtration can remove excess fractionating agent (e.g.,polymer) which is not attached to CNTs, thus allowing a polymer:CNTstoichiometry to be determined. Typically, the solutions are filteredthrough a 4-4.5 micron (μm) sintered glass filter and then washedrepeatedly with a solvent. The CNTs attached to a polymer that areisolated by filtration in this manner are either characterized as-is orredissolved in a solvent via water bath sonication (5-60 minutes, mosttypically 30 minutes), and then characterized.

In some embodiments, a step of isolating the CNTs from the supernatantis performed. In some embodiments, the CNTs are freed of fractionatingagent and/or other materials via washings. In some embodiments, theisolated CNTs are mixed with other materials (e.g., polymer or polymerprecursor materials) prior to being transferred to the substrate.

In some embodiments, the centrifugation/fractionating step and/or theoptional filtration step can be repeated up to numerous times, eachsuccessive iteration leading to a supernatant more thoroughly enrichedin semiconducting CNTs. Ultimately, if enough iterations are employed,isolated semiconducting CNTs could be obtained.

In some embodiments, the transferring requires an application process.Typically, such application processes are solution-based. Suitable suchprocesses include, but are not limited to, spraying, spin-coating,brushing, rolling, printing, inkjet printing, screen printing, andcombinations thereof. Solution-based transfer typically requires asolvent removal step. Typically, such solvent removal employs anevaporative means.

As mentioned above, FET devices of the present invention utilizing CNTsare typically TFT devices. In some embodiments, carbon nanotubes (CNT)are used as the active semiconductor material (or as a component of suchmaterial) in a thin film transistor (TFT) device that can be fabricatedusing simple, solution-based processing that permits the use of plasticsubstrates. Key to these devices, and the processes by which they aremade, are the processes by which metallic CNTs can either be segregated,fractionated, destroyed, or even removed from the CNT population used toform the network such that the TFTs do not short circuit due to the highconductivity of the metallic CNTs.

Metallic and semiconducting CNTs may be segregated by exfoliating CNTbundles using surfactants or dispersants such as conjugatedoligomers/polymers (see above). In some embodiments, these dispersantsmay further allow the physical separation (e.g., fractionation) ofmetallic from semiconducting CNTs, thus facilitating device fabrication.Selective destruction of the metallic CNTs may also be employed in orderto obtain a CNT network that is more largely semiconducting. Selectivedestruction may be achieved chemically using known approaches such asreaction with aryl diazonium salts (Bahr et al., “Functionalization ofCarbon Nanotubes by Electrochemical Reduction of Aryl Diazonium Salts: ABucky Paper Electrode,” J. Am. Chem. Soc. 2001, 123, 6536-6542).

The present invention provides a low-cost process that is solution-basedand can be done at relatively low temperatures, and, as a result, iscompatible with plastic substrates. In some exemplary embodiments, thepresent invention is directed toward devices based on SWNT networks thatare obtained from SWNTs manufactured using large-scale productiontechniques such as the HiPco process (Nikolaev et al., “Gas-PhaseCatalytic Growth of Single-Walled Carbon Nanotubes from CarbonMonoxide,” Chem. Phys. Lett., 1999, 313, 91-97). These tubes are amixture of metallic (˜30%) and semiconducting (˜70%) CNTs. In someembodiments, it is these CNTs that are dispersed in solution andfractionated into populations of CNTs enriched in metallic CNTs andsemiconducting CNTs, so as to provide for a semiconducting network ofCNTs for use in FET devices. Such CNT populations enriched insemiconducting CNTs can be further treated chemically in order toselectively render some or all of the remaining metallic tubesnon-conducting, resulting in a more largely semiconducting CNT network.See, e.g., L. An et al., “A Simple Chemical Route to SelectivelyEliminate Metallic Carbon Nanotubes in Nanotube Network Devices,” J. Am.Chem. Soc., 2004, 126(34), 10520-10521.

It is worth noting that the use of silicon devices on plastic is limitedto amorphous silicon (deposited chemically), which limits theperformance of such devices on plastic (mobility ˜1 cm²/Vsec). Thisproblem is addressed herein by using CNTs, such as in the form of SWNTnetworks, which can provide mobilities generally greater than about 10cm²/Vsec and more typically up to about 100 cm²/Vsec due to theballistic nature of transport in SWNTs. This would allow fabrication ofall of the circuitry in a liquid crystal display (LCD) using SWNTdevices.

In some embodiments, the present invention is directed to processes bywhich TFTs based on SWNT networks are fabricated. SWNT dispersions insolution are used to fabricate TFTs (using solution-based processing) inwhich the semiconducting channel of the TFT is a SWNT network. The firstprocess relies on dispersions that are highly optimized such thatsegregating and/or fractionating metallic SWNTs from semiconducting SWNTyields a suitably semiconducting network upon deposition in a TFTchannel. The use of conjugated polymers/oligomers allows and facilitatesthe dispersion of SWNTs such that SWNT bundles (containing both metallicand semiconducting SWNTs) can be exfoliated into individual SWNTs, andprovides for fractionation of such metallic and semiconducting SWNTsfrom each other upon centrifuging, thus resulting in a semiconductingSWNT network upon deposition onto a substrate. The channel length canalso be adjusted to assure that no individual tube spans its length,thus precluding a metallic tube, either alone or in a bundle, fromshort-circuiting the TFT. Other types of dispersants are also envisioned(surfactants and other non-polymeric weakly associated species, oftenbeing water soluble). The substrate may be a plastic substrate.

In some embodiments, selective reagents are employed to further modifythe electronic properties of the metallic SWNT (i.e. render themnon-conducting), and such is another process by which one can furtherobtain a semiconducting network of SWNT in the TFT channel. This may bedone while dispersing the SWNT in solution, or on the TFT devicedirectly, after the SWNT network has been formed in the device channel.This process may be combined with the use of appropriate dispersants.Examples of covalent chemistry capable of destroying metallic SWNTsinclude reaction with aryl diazonium salts (see above).

Another process involves the dispersion of a semiconducting SWNT (orother CNT) network into a semiconducting polymer matrix such aspolythiophene. This approach may facilitate the charge injection fromthe source and drain electrodes into the channel and subsequently intothe SWNT. This process may also be combined with SWNT physicalseparation or selective chemistry to destroy the metallic tubes prior todispersing them in the semiconducting polymer.

A central feature of the present invention is the ability to makesemiconducting SWNT networks from pristine SWNTs exposed tofractionation treatments and selective chemical destruction of themetallic CNTs. As such selective chemical destruction is not completelyselective, the combination of these two treatments affords tremendousflexibility in the processing and higher quality semiconductor activematerials that comprise such semiconducting CNTs. Furthermore, theintroduction of the CNTs into a semiconducting polymer matrix, ordirectly onto a substrate, permits the fabrication of better-performingTFTs than traditional organics or amorphous silicon can provide. Thepresent invention thus provides a low-cost, solution-based process formaking TFTs while not sacrificing the performance of the TFT (e.g.,mobilities of CNT-based TFTs can be greater than 100 cm²/Vsec).

EXAMPLES

The following examples are provided to demonstrate particularembodiments of the present invention. It should be appreciated by thoseof skill in the art that the methods disclosed in the examples whichfollow merely represent exemplary embodiments of the present invention.However, those of skill in the art should, in light of the presentdisclosure, appreciate that many changes can be made in the specificembodiments described and still obtain a like or similar result withoutdeparting from the spirit and scope of the present invention.

Example 1

This Example serves to illustrate how CNTs can be selectivelyfractionated, according to some embodiments of the present invention, bytype to yield CNT populations enriched in semiconducting CNTs.

CNTs used in this Example were HiPco-produced SWNTs of purified gradeobtained from Carbon Nanotechnologies, Inc. (Houston, Tex.). Suchpurified tubes comprised metal impurities (predominantly Fe catalyst) ina range of 2-20 weight percent. These SWNTs typically have diameters inthe range of 0.7-1.3 nm, lengths in the range of 1-1.5 microns, and anaverage electronic bandgap of around 0.8 eV.

Referring to FIG. 4, such fractionation typically requires three steps:(Step 4001) solubilization, (Step 4002) centrifugation, and (Step 4003)filtration. These steps are described in more detail below.

(Step 4001) Solubilization: In vial 1, 0.1-1 mg/ml of purified SWNTswere partially dispersed in a solvent, most typically CHCl₃, bysonicating the vial in a water bath for 5-30 minutes, most typically 15minutes. The total weight of SWNTs used per experiment never exceeded100 mg. The respective solubilization/fractionation agent was dissolved(by sonication in water bath) in vial 2 containing the same solvent. Theweight of the fractionating agent used was such that the resultingfractionation agent:SWNT ratio ranged from 0.1:1 to 20:1 by weight, butmost was typically about 1:1. The contents of vial 2 were added to vial1 and the mixture was sonicated for between 15 minutes and 3 hours, mosttypically about 30 minutes. The resulting solution of wholly orpartially dispersed SWNTs was then processed via purification Step 4002and/or Step 4003 below.

(Step 4002) Centrifugation: The solution from Step 4001 was centrifugedfor 2-15 minutes, most typically 5 minutes, at a speed of between about1000 to 5000 rpm, most typically 3500 rpm. The sediment (precipitate)was separated from the supernatant (eluant) and the latter was processedeither in accordance with Step 4003 below, or submitted directly forcharacterization. The precipitate either was characterized as-is orre-dissolved via water bath sonication (5-60 minutes, most typically 30minutes), in a solvent, most typically CHCl₃, and then characterized byRaman spectroscopy. This served as a means of separating, in whole or inpart, dispersed SWNTs from bundled SWNTs.

(Step 4003) Filtration: Solutions obtained directly from Step 4001, orafter processing according to Step 4002, were filtered through a 4-4.5micron sintered glass filter and then washed repeatedly with the samesolvent (free fractionating agent was highly soluble in this solvent).The SWNTs isolated by such filtration were either characterized as-is orredissolved via water bath sonication (5-60 minutes, most typically 30minutes), the solvent most typically being CHCl₃, and then characterizedusing Raman spectroscopy and ultraviolet/visible/near-infrared(UV/vis/NIR) spectroscopy. This served as a means of removing excessfractionating agent from the SWNTs and thus allowing the determinationof the fractionating agent:SWNT stoichiometry.

Example 2

This Example serves to illustrate how Raman spectroscopy can be used toconfirm fractionation of CNTs.

Evidence for the selective solubilization of semi-conducting SWNTs wasobtained from Raman studies using a 514 nm laser light source, as shownin FIG. 5. The relative intensities of the bands in spectrum C(“centrifuge ppte,” i.e., post-centrifuge sediment, in FIG. 5) aresignificantly different from the intensities in the other three spectrashown in FIG. 5. This comparison suggests that P3HT partially separatesSWNTs into semiconducting (sc)-enriched (supernatant) and metallic(met)-enriched (sediment) portions. Analysis of the Raman data was donein accordance with previous analyses (H. Katahura et al., “OpticalProperties of Single-Wall Carbon Nanotubes,” Synthetic Metals, 1999,103, 2555-2558; M. Dresselhaus et al., “Phonons in Carbon Nanotubes,”Advances in Physics, 2000, 49, 705-814).

While not intending to be bound by theory, Table 1 is a summary andinterpretation of the radial breathing mode (RBM) region of the 514 nmRaman data obtained for soluble P3HT-SWNT dispersions solvent-cast ontoquartz wafers from CHCl₃ solutions. The spectrum obtained for pristine,or untreated (i.e., no moiety has been attached), SWNTs indicates thepresence of both semiconducting (sc) and metallic (met) SWNTs, with theintensities of the peaks of both the sc-SWNTs and met-SWNTs beingrelatively strong. In the spectra obtained for both the P3HT-SWNTdispersion and the supernatant (“supernatant” in Table 1), the met-SWNTpeaks have decreased relative to the intensity of the sc-SWNT peaks,indicating that the supernatant is enriched in sc-SWNTs as a result ofthe selective functionalization and dispersion of the sc-SWNTs. In thespectrum obtained for the sediment, the situation is reversed, as themet-SWNT peaks are relatively stronger than the sc-SWNT peaks. Thisindicates that the sediment is enriched with met-SWNTs and, conversely,that the sc-SWNT concentration in the sediment is depleted.

Raman studies using a 633 nm laser source provided further evidence forselectively dispersing semi-conducting SWNTs. FIG. 6 depicts the radialbreathing mode (RBM) region of the spectra obtained. These findingsstrongly support selective solubilization of semi-conducting SWNTs(especially with diameters in the vicinity of 0.9 nm) and the resultingseparation into an sc-enriched soluble phase and a met-enrichedinsoluble phase. Again, while not intending to be bound by theory, Table2 is a summary and interpretation of the RBM region of the 633 nm Ramandata for polymer P3HT-SWNT dispersions solvent-cast onto quartz wafersfrom CHCl₃ solutions.

TABLE 1 P3HT- RBM band SWNT Pristine SWNT position Diameter SWNTsdispersion Sediment Supernatant (+/−2 cm⁻¹) (+/−0.03 nm) intensityintensity intensity intensity Assignment 183 1.32 m w w w sc 203 1.18 ss m-s s sc 227 1.05 w w-m w w met 246 0.96 m w-m s w-m met 261 0.90 mw-m s w-m met

TABLE 2 P3HT- SWNT Pristine SWNT RBM band diameter SWNTs dispersionSediment Supernatant (+/−2 cm⁻¹) (+/−0.03 nm intensity intensityintensity intensity Assignment 190 1.26 m, sh m, sh m, sh — met 202 1.18m, sh m, sh m, sh w met 212 1.12 s s s m met 218 1.09 — — — m met 2530.93 m m m s sc w = weak; m = medium; s = strong; sh = shoulderby the significantly narrower and weaker profile of the band envelope inthe 1540-1600 cm⁻¹ region (and the band's slight blue shift) resultingfrom the soluble phase (trace 7D) compared to the other three tracesshown. This is consistent with the supernatant (soluble phase) having asignificantly higher relative content of sc-SWNTs than any of the otherthree samples. Accordingly, the insoluble phase (trace 7C) has thehighest met:sc SWNTs ratio. This further supports the selectivesolubilization of sc-SWNTs, and the resulting separation into ansc-enriched soluble phase and a met-enriched insoluble phase.

Example 3

This Example serves to illustrate a manner in which UV/vis/NIRspectroscopy can be used to confirm fractionation of CNTs.

In one particular embodiment, carbon nanotubes are dispersed byattaching at least one conjugated soluble polymer such as apolythiophene, polythiophene derivatives, a polyfluorene, orcombinations thereof to the nanotube. In a particular embodiment, theconjugated soluble polymer is P3HT. While not intending to be bound bytheory, the enhanced solubilization of some SWNTs by non-covalentlyattaching P3HT to SWNTs is shown by the UV/vis/NIR absorption spectra inFIG. 8. The UV/vis/NIR absorption spectra in FIG. 8 shows that P3HT hasan affinity for the SWNTs, as evidenced by the 550-650 nm range (circledarea FIG. 8). For comparative purposes, an UV/vis/NIR absorptionspectrum of untreated SWNTs is shown in FIG. 9

Example 4

This Example serves to illustrate how solution-based processingtechniques can be used to fabricate a TFT device comprising CNTs.

Referring to FIG. 10, the processing begins with a silicon (Si) wafer1001 (comprising an ˜1000 Å silicon oxide layer 1008) on which a mask isapplied to pattern source and drain electrodes 1002 and 1003,respectively. SWNTs are then solution cast onto the wafer such that theyfill the channel region 1004 between the source and drain electrodes.Then, a layer of dielectric material 1005 is deposited. Finally, a gatematerial 1006 is deposited on the dielectric layer 1005 to yield a TFTdevice 1007.

Example 5

This Example serves to illustrate a testing device suitable for testingand/or screening semiconducting active materials for use in TFT devices.

Referring to FIG. 11, test device 1100 has dimensions of 1000 μm×1000μm. The first patterned material is metal, illustrated in black. Testdevice 1100 comprises a guard ring 1101, a source electrode 1102, adrain electrode 1103, and a channel region 1104 having a channel length“L” and a channel width “W.” The channel region is the area between thesource and drain electrodes. The channel length is defined as thedistance between the source and drain electrodes and the channel widthis defined as the width of the drain electrode. Typically, in suchtesting devices, the channel length L ranges from about 2 μm to about100 μm, and the channel width ranges from about 10 μm to about 1000 μm.The square pads are probe pads 1107, with dimensions of 100 μm×100 μm.All metal lines are 10 μm in width. The metal can be 500 Å molybdenum,platinum, or gold. Usually, 100 Å of titanium is put beneath to promoteadhesion. Region 1200, identified in FIG. 11, is shown in across-sectional side view in FIG. 12. Referring to FIG. 12, the firstpatterned layer on substrate 1201 comprises source and drain electrodes1102 and 1103, respectively, and guard ring 1101. The second patternedlayer is dielectric material 1105. The dielectric materials can be SiO₂,parylene, etc. The third patterned layer, 1106, is the gate layer.

Example 6

This example serves to illustrate how mobility can beincreased/optimized by modulating the CNT density within thesemiconducting active material.

An intuitive way of increasing mobility within a CNT-based TFT device isto increase the CNT density within the semiconducting active region(i.e., the channel region). By doing this, however, the possibility offorming continuous metallic paths that bridge the source and drainelectrodes also increases. Using multiple fractionation steps involvingP3HT suspended SWNTs, such fractionated SWNTs were incorporated intodevice 1100 with a density of about 10 nanotubes per μm². Thisapproximate density is illustrated visually in FIG. 13, which is aatomic force micrograph (AFM) of such nanotubes on a surface. Thisdensity of SWNTs within device 1100 yielded a mobility of around 5cm²/Vsec, as shown in the transfer curve of FIG. 14.

Example 7

This Example serves to illustrate a manner in which on/off ratios can beobtained with devices in which SWNTs have been spun cast into thechannel region from a DMF suspension.

DMF-suspended SWNTs were spun cast into the channel region of device1100. The resulting device is shown in the scanning electron micrographs(SEMs) of FIGS. 15 and 16, wherein FIG. 16 is a higher magnification ofa region of the area shown in FIG. 15. With such devices, on/off ratiosof 10⁵ and mobilities of 0.1 cm²/Vsec were achieved, as shown in FIG.17. It is worth noting, however, that, as seen in the SEMs of FIGS. 15and 16, the density of SWNTs within the device is low.

In conclusion, the present invention provides field effect transistorscomprising carbon nanotubes made by solution based processingtechniques, wherein the carbon nanotubes within such devices have beenfractionated so as to be concentrated in semiconducting carbonnanotubes. Additionally, the relatively low temperature solution-basedprocessing methods enable the use of plastics, and other temperaturesensitive materials, in the fabricated devices.

It will be understood that certain of the above-described structures,functions, and operations of the above-described embodiments are notnecessary to practice the present invention and are included in thedescription simply for completeness of an exemplary embodiment orembodiments. In addition, it will be understood that specificstructures, functions, and operations set forth in the above-describedreferenced patents and publications can be practiced in conjunction withthe present invention, but they are not essential to its practice. It istherefore to be understood that the invention may be practiced otherwisethan as specifically described without actually departing from thespirit and scope of the present invention as defined by the appendedclaims.

1-15. (canceled)
 16. A field effect transistor comprising: a) a plasticsubstrate; b) a gate electrode; c) a dielectric layer in contact withthe gate electrode; d) a semiconducting active material in contact withthe dielectric layer, wherein the semiconducting active materialcomprises carbon nanotubes, the carbon nanotubes having beennon-destructively enriched in semiconducting carbon nanotubes; and e)source and drain electrodes in contact with the semiconducting activematerial.
 17. The field effect transistor of claim 16, wherein thesemiconducting active material further comprises material selected fromthe group consisting of polymeric material, oligomers, single-strandedDNA, polyethylenimine, C60, aromatic organic molecules, and combinationsthereof.
 18. The field effect transistor of claim 16, wherein thesemiconducting active material further comprises a polymeric material incontact with the carbon nanotubes.
 19. The field effect transistor ofclaim 18, wherein the polymeric material has dendritic material attachedto it.
 20. The field effect transistor of claim 16, wherein thesemiconducting active material further comprises a polythiophenematerial.
 21. The field effect transistor of claim 16, wherein thetransistor realizes charge carrier mobility in the semiconductor activeregion that is between about 10-3 cm2/Vsec and about 105 cm2/Vsec. 22.The field effect transistor of claim 16, wherein the transistor realizescharge carrier mobility in the semiconductor active region that isbetween about 10-1 cm2/Vsec and about 30 cm2/Vsec.
 23. The field effecttransistor of claim 16, wherein the carbon nanotubes within thesemiconducting active material have a density high enough to achieve apercolation threshold.
 24. The field effect transistor of claim 16,wherein the transistor is part of a matrix-addressable array, with eachnode in the array comprising at least one such transistor.
 25. A fieldeffect transistor comprising: a) input and output electrodes; and b) asemiconducting active material coupled to the input and outputelectrodes, wherein the semiconducting active material comprises carbonnanotubes, the carbon nanotubes having been non-destructively enrichedin semiconducting carbon nanotubes.
 26. The field effect transistor ofclaim 25, wherein the substrate is a polymeric material.
 27. The fieldeffect transistor of claim 25, wherein the semiconducting activematerial further comprises material selected from the group consistingof polymeric material, oligomers, single-stranded DNA, polyethylenimine,C60, aromatic organic molecules, and combinations thereof.
 28. The fieldeffect transistor of claim 25, wherein the semiconducting activematerial further comprises a polythiophene material.
 29. The fieldeffect transistor of claim 25, wherein the transistor realizes chargecarrier mobility in the semiconductor active region that is betweenabout 10-3 cm2/Vsec and about 105 cm2/Vsec.
 30. The field effecttransistor of claim 25, wherein the transistor realizes charge carriermobility in the semiconductor active region that is between about 10-1cm2/Vsec and about 30 cm2/Vsec.
 31. The field effect transistor of claim25, wherein the carbon nanotubes within the semiconducting activematerial have a density high enough to achieve a percolation threshold.32. The field effect transistor of claim 25, wherein the transistor ispart of a matrix-addressable array, with each node in the arraycomprising at least one such transistor.
 33. A field effect transistorcomprising: a) a plastic substrate; b) a gate electrode; c) a dielectriclayer in contact with the gate electrode; d) source and drainelectrodes; and e) a semiconducting active material bridging the sourceand drain electrodes, the material comprising a nanotube network ofsemiconducting and metallic carbon nanotubes, wherein the nanotubenetwork has a nanotube density sufficient to achieve percolationthreshold and below a level that would short the device.
 34. The fieldeffect transistor of claim 33, wherein the semiconducting activematerial further comprises material selected from the group consistingof polymeric material, oligomers, single-stranded DNA, polyethylenimine,C60, aromatic organic molecules, and combinations thereof.
 35. The fieldeffect transistor of claim 33, wherein the semiconducting activematerial further comprises a polythiophene material.
 36. The fieldeffect transistor of claim 33, wherein the transistor realizes chargecarrier mobility in the semiconductor active region that is betweenabout 10-3 cm2/Vsec and about 105 cm2/Vsec.
 37. The field effecttransistor of claim 33, wherein the transistor realizes charge carriermobility in the semiconductor active region that is between about 10-1cm2/Vsec and about 30 cm2/Vsec.
 38. The field effect transistor of claim33, wherein the transistor is part of a matrix-addressable array, witheach node in the array comprising at least one such transistor.